D2D (device to device) is a new topic in 3GPP LTE release 12, and the main target for such study item is to realize direct device-to-device communication. D2D communication could happen within network coverage (for commercial case) and without network coverage (for public safety).
FIG. 1 is a schematic diagram showing two D2D communication scenarios. As shown in FIG. 1, in a scenario 100A shown in the left part of FIG. 1, two wireless devices 101 and 102 realize direct device-to-device communication, within network coverage by eNode B 103; while in the other scenario 100B shown in the right part of FIG. 1, two wireless devices 104 and 105 realize direct device-to-device communication without network coverage.
In Rel. 12, the focus on D2D communication is mainly out-of-network coverage scenario and broadcasting traffic.
One issue on out-of-network coverage scenario is the structure of D2D communication. Currently there are two candidates for the structure of D2D communication basically: 1) centralized structure as shown in FIG. 2A; and 2) distributed structure as shown in FIG. 2B.
FIGS. 2A and 2B are schematic diagrams showing a centralized structure and a distributed structure in D2D communication, respectively.
In FIG. 2A, the solid line represents the data signal and the dashed line represents the control signal. It can be seen that there are two kinds of wireless device (which may be also referred to as user equipment, UE), the cluster head (or master UE) 201 and the slave UEs 202A-202D, in the centralized scenario. Signaling is controlled by the cluster head, but data could be directly transmitted from a slave UE to another slave UE in such scenario.
In FIG. 2B, similarly, the solid line represents the data signal and the dashed line represents the control signal. It can be seen that, there is no definition of cluster head (or master UE) and slave UE in the distributed scenario. The identifications of all the UEs 203A-203E are equal. The control signaling and data are both transmitted from a transmitting UE to a receiving UE.
Another issue on D2D communication is the power control problem. Currently, there is no power control based on most companies' understanding. Thus a maximum power transmission is the basic assumption. This would cause large power consumption and interference to other UEs.
FIG. 3 is a schematic diagram showing the problem caused due to the maximum power transmission. As shown in FIG. 3, it is desirable for a transmitting UE 301 to transmit data and signaling to the receiving UEs 302A-302C in a cluster to which the transmitting UE 301 belongs. Thus, the optimal transmission power range is as shown by the ellipse in the dashed line in FIG. 3. However, the maximum transmission power range is as shown by the ellipse in the solid line in FIG. 3. Therefore, it causes not only a large power consumption of the transmitting UE 301, but also a large interference to the non-targeted UEs 303A and 303B.